The long-range purpose of the present project is to investigate the role of retroviruses in the etiology of reticulum cell as well as other lymphoid cell neoplasia, and to study the host responses, functional markers and surface antigens of tumor cells. The topics of current interest are: 1) Characterization of xenotropic retroviruses isolated from SJL/J strain of mouse which exhibit a high incidence of spontaneous reticulum cell neoplasm (RCN); and investigation of the role played by these viruses. 2) Isolation and characterization of a new dual tropic retroviruses from Abelson virus-induced lymphoma in BALB/c mice. 3) Natural killer cell activity exhibited by transplantable RCN of SJL/J mouse. 4) Tumor specific transplantation antigen(s) of RBL-3 (Rauscher virus-nonproducer) and RBL-5 (Rauscher virus-producer) cells. 5) Production of anti-viral and anti-FOCMA (feline oncovirus-associated cell membrane antigen) antibodies in guinea pigs.